I've loved The Office (American), but this last season or two it's really gone downhill. Not enough conflict, not enough misery (especially with Jim and Pam). The happiness of real people is important, and we should care about that. And we tend to translate that onto fictional characters. We identify with them, their suffering, their happiness, and so we want them to be successful and happy. But happiness is what real people deserve. As writers, we have to resist that, because it's not interesting, not for very long. Some might think they want all the characters to find true love and self fulfillment. But they don't want that, really, because it kills the story.
A successful story either has to end at the point of triumph ("and they lived happily ever after"), or, if it keeps going, some new difficulty has to arise. This doesn't necessarily have to be the same type of difficulty as before, the undoing of whatever success has already come, but it does have to arise, and to seem urgent and compelling, rather than a comparatively superficial problem on top of overall serenity.
And this, I think, is one of the difficulties with sitcoms. A movie has a discrete end. So does a book. The author can more easily chop things off at a satisfying point. But a sitcom has a tendency to keep going and going and going for as long as there's a sufficient fanbase and sufficient money. (Of course the same thing happens with books and movies, when there's more sequel than there is compelling material.) Arcs complete themselves, the narrator's lives progress. Unless the sitcom is one of those time-frozen ones, where everything returns to the status quo at the end (like The Simpsons), it eventually becomes unrealistic to stay with the same sources of dramatic tension, and it becomes boring to keep inventing slightly different obstacles to the same outcome. Either the couple gets together, or they get over it. Or we get a Friends type clusterfuck, where the same damn people get together and break up over and over and then all get married to one another in a big circle-jerk of fan-pandering. Much preferable is, say, the series ending of Seinfeld, where all the show's main characters--unredeemable and dislikable, but always interesting--die in a plane crash.
I think the character of Vimes, in Pratchett's Discworld series, is an example of an author successfully handling the cumulative triumphs of a character, while constantly keeping problems and danger imminent, and stakes high.
In Vimes' first appearance, in the novel, Guards! Guards!, he is a misanthropic, despairing alcoholic, in an underpaid, Sisyphean, and irrelevant job as a City Watchman. Over the course of several novels, he becomes happily married to the richest woman in the city, thus becoming the richest man in Ankh-Morpork; rises in rank to the Commander of the Watch and then to Duke, expands and revolutionizes the institution of the City Watch, and gains international influence and respect.
But while you're glad for him and glad to see the progress that he and the Watch make, there's no placidity, no happily-ever-after zone where the characters are just sculling around, and we're still watching them because we've grown to like them and want to see them content. (See: Jim and Pam, season 5. Even the potential minor conflicts are usually resolved by the end of an episode!) With every one of Vimes' gains comes a challenge of greater scope, and failure always has dire consequences, for entire cities and populations as well as for Vimes personally. This is one way in which Pratchett keeps it interesting, even while the characters--and perhaps the city as a whole--continually succeed and grow in stature and comfort by the end of every book.
That is mostly to do with the plot. Equally critical to Pratchett's successful handling of this is his characters' personalities and attitudes, and the realism (or cynicism) of both the characters and the author. Vimes is still a misanthropic alcoholic (though later a dry one), and feels uneasy with material comfort and prestige. This sets up a dramatic conflict between the character and his own successes. Failure and misery? Always dramatic. Success? In Vimes' case, also a source of frisson. This is a character whose happiest moments involve skulking in the rain at 3 am and chasing an armed thief down an alley, who chafes at reward, and who can't help but stir the turd. In other words, we are, as readers, guaranteed a good time.
As for the author's cynicism--well, you don't want to simply take a look at someone's books and automatically infer things about their beliefs and worldview, but the expository voice, and the nature of events as they unfold in his books, is pretty consistent. It would also be overly simplistic to characterize Pratchett's worldview as simply "cynical," as if it just despaired of or belittled humanity. His perspective is much more nuanced, clear-eyed, understanding, and forgiving than that.
Humans are foolish, brutal, selfish, ignorant, small-minded, curious, inventive, resourceful, adaptable, insufferable, and creative. They can make gods out of the desert wind, boredom out of the majesty of the universe, and a damn mess out of anything with a proverbial DO NOT TOUCH sign. The kind of happiness you find, on Discworld, is the kind that's Good Enough. War and conflict may be on the horizon, but for now at least we have peace. Problems are forestalled, not eradicated, and we can rest at the end of a satisfying narrative, applaud the characters' successes, while well assured that something else will come along and fuck things up, and we'll be treated to another adventure.